The role of membrane-bound iron-sulfur proteins as carriers in electron transfer reactions will be investigated. The relationship of these proteins to primary photochemical events and to energy conservation will be the major emphasis of these studies. EPR spectroscopy at low temperature will be used to observe the iron-sulfur proteins in photosynthetic systems after illumination of samples at physiological temperatures. The aim of these studies is to clarify the role of iron-sulfur proteins as oxidation-reduction carriers in membrane-bound systems and to explore the possible role of these carriers in electron transfer reactions which are coupled to phosphorylation.